The mechanisms of visual adaptation and wavelength discrimination at the retinal level are studied in fish (carp, catfish). Electroretinographic or intracellular potentials are recorded from isolated retinas maintained in a perfusion chamber. The effects of the spatial configuration and the wavelength of photopic stimuli upon the visual threshold of retinal neurons is investigated in different states of dark or light adaptation. Parallel anatomical studies are initiated whose aim is to identify the kinds and numbers of synaptic contacts made between retinal neurons.